holds sndstat_lock across a call to uiomove(), which is not legal
to do with a mutex because of the possibility that the data transfer
could sleep because of a page fault. It is not possible to just
unlock the mutex for the uiomove() call without introducing another
locking mechanism to prevent the body of sndstat_read() from being
re-entered. Converting sndstat_lock to an sx lock is the least
complicated change.
This is a candidate for RELENG_5.
LOR: 030
MFC after: 4 days